2019 ASRJC Prelim H2 Chem P2 ANS

ASRJC JC2 PRELIMS 2019 9729/02/H2 ANDERSON SERANGOON JUNIOR COLLEGE H2 Chemistry 9729 2019 JC2 Prelim Exam Paper 2 Solutions 1 (a) Reducing power of Group 2 elements increases [1] down the group. Number of (principal) quantum shell / electronic shell increases and valence electrons are further away and more shielded from the nucleus. Valence electrons are less strongly attracted to the nucleus and smaller amount of energy is needed to remove the valence electron so it is more easily oxidised. [1] (b) (i) Copper can give up both 3d and 4s electrons for delocalisation whereas calcium can only give up 2 electrons . [1] Electrons act as mobile charge carriers [1] to conduct electricity. (ii) Cu has smaller atomic radius and higher atomic mass [1] compared to Ca. Also, Cu has a more closely packed structure due to stronger metallic bonding as compared to Ca. [1] Hence, Cu has a much higher density than Ca. (c) (i) A Each orbital can hold a maximum of 2 electrons of opposite spin. Diagram B has a pair of electrons having the same spin. [1] with explanation (ii) 3𝑑𝑧2 [1] 3𝑑𝑥𝑧 [1] Alternative answers: 3𝑑𝑥2−𝑦2, 3dxy, 3dyz (d) (i) Both silicon and silica have a giant molecular structure. [1] (ii)  From the data booklet: Si–Si BE: 222 kJ mol–1 Si–O BE: 460 kJ mol–1 [1]  Size of valence orbitals: Si > O  Valence orbitals of Si are more diffused than that of O  Overlap of orbitals is less effective in Si–Si than in Si–O  The Si–O bond in silica is stronger than the Si–Si bond in silicon.  Hence, bond energy: Si –Si < Si–O and more energy is required to break the Si–O bond than the Si–Si bond. [1]

ASRJC JC2 PRELIMS 2019 9729/02/H2 2 2 (a) (i) presence of carboxylic acid or –COOH group presence of CH3CO– or methyl ketone absence of chiral carbon any 2 for [2] (ii) C D CH3 C O CH2CH2COOH [1] each (iii) E: (CH3)2CO; F: CH3CH2CHO [1] each (iv) G: CH3CH=CHCH(CH3)2 [1] (b) (i) Br Br Br Br [1] Orange–red / red–brown Br2 decolourises / turns colourless. [1] (ii) NaOH in ethanol, heat (under reflux) [1] (iii) BrBr [1] 3 (a) (i) [1]: diagram As shown on the diagram, in the presence of catalase, at a certain temperature T,  Catalase provides a different pathway of a lower activation energy (Ea’ < Ea).  More molecules have energies greater than or equal to the lowered activation energy Ea’.  This results in an increase in the frequency of effective collisions. [2] Ea Ea’ Number of molecules with energy Energy Represents n